In commonly owned U.S. Pat. No. 3,553,966, granted to Karl-Heinz Liebert, there has been disclosed an automotive steering system of this type wherein the flow of hydraulic fluid (referred to hereinafter as oil) from a main pump to a servomotor and an associated ancillary or metering pump is controlled by a valve structure forming two cylinders for a primary and a secondary valve piston which are manually displaceable in opposite directions by the driver of the vehicle. The primary valve piston has two pairs of peripheral grooves, i.e. an inner pair on opposite sides of a central plane of symmetry and an outer pair disposed between these inner grooves and the piston ends, these grooves being defined by axially spaced piston heads whose peripheral surfaces coact with the peripheral cylinder surfaces in establishing various flow paths. In the normal, centered position of the piston, its inner grooves communicate with the low-pressure side of the oil source, constituted by the aforementioned main pump and an associated reservoir or sump, and also with the high-pressure side of that source so that the oil is continually and ineffectually recirculated; the outer grooves are then cut off from the source. These outer grooves, in turn, open toward a pair of channels which lead to the servomotor and the ancillary pump via the secondary valve cylinder. Moreover, the inner grooves of the primary valve piston give access to respective axial bores in the piston which are partially occupied by a pair of fixed or spring-loaded plungers designed, upon a shifting of that piston into an off-center position, to exert a reaction force giving the driver a simulated feeling of road resistance even as the servomotor is actuated to steer the vehicle to one or the other side. These two axial bores may thus be regarded as branches of a hydraulic load circuit also including the two channels which extend to the servomotor and the ancillary pump.
In systems of this or a similar nature it has heretofore been the practice, as also shown in the Liebert patent, to provide the piston heads with transverse lands bounding the intervening grooves, these lands forming flow-controlling edges which together with similar cylinder edges instantaneously block or unblock the associated oil passages upon a shifting of the piston. As the piston and cylinder edges approach each other to throttle the flow upon incipient closure of a passage, the oil experiences a sharp deflection in a radial direction which may give rise to annoying and therefore undesirable whistling noises.